Grabby Aliens

Advanced aliens really are out there, and we have enough data to say roughly where they are in space and time, and when we will meet or see them.

Grabby Aliens Plot 2


There are two kinds of alien civilizations. “Quiet” aliens don’t expand or change much, and then they die. We have little data on them, and so must mostly speculate, via methods like the Drake equation.

“Loud” aliens, in contrast, visibly change the volumes they control, and just keep expanding fast until they meet each other. As they should be easy to see, we can fit theories about loud aliens to our data, and say much about them, as S. Jay Olsen has done in 7 related papers (1, 2, 3, 4, 5, 6, 7) since 2015.

Furthermore, we should believe that loud aliens exist, as that’s our most robust explanation for why humans have appeared so early in the history of the universe. While the current date is 13.8 billion years after the Big Bang, the average star will last over five trillion years. And the standard hard-steps model of the origin of advanced life says it is far more likely to appear at the end of the longest planet lifetimes. But if loud aliens will soon fill the universe, and prevent new advanced life from appearing, that early deadline explains human earliness.

“Grabby” aliens is our especially simple model of loud aliens, a model with only 3 free parameters, each of which we can estimate to within a factor of 4 from existing data. That standard hard steps model implies a power law (t/k)n appearance function, with two free parameters k and n, and the last parameter is the expansion speed s. We estimate:

  • Expansion speed s from fact that we don’t see loud alien volumes in our sky,
  • Power n from the history of major events in the evolution of life on Earth,
  • Constant k by assuming our date is a random sample from their appearance dates.

Using these parameter estimates, we can estimate distributions over their origin times, distances, and when we will meet or see them. While we don’t know the ratio of quiet to loud alien civilizations out there, we need this to be ten thousand to expect even one alien civilization ever in our galaxy. Alas as we are now quiet, our chance to become grabby goes as the inverse of this ratio.


Here is our main academic-style paper on the subject:

“If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare” by Robin Hanson, Daniel Martin, Calvin McCarter, and Jonathan Paulson, March 25th, 2021.

The hard-steps model of advanced life timing suggests humans have arrived early. Our explanation: “grabby” civilizations (GC), who expand fast and long, and change their volumes’ appearances, set an early deadline. If we might soon become grabby, today is near a sample GC birthdate. Fast GC expansion explains why we do not see them. Each of our three model parameters is estimable from data, allowing detailed GC predictions. If GCs arise from non-grabby civilizations (NGCs), a depressingly low transition chance (~10-4) seems required to expect even one other NGC ever active in our galaxy.

(See also prior version “A Simple Model of Grabby Aliens”, February 1, 2021.)

Presentations and interviews

How Far Away Are Expansionist Aliens?, SciFuture, December 28th, 2020.

A Simple Model of Grabby Aliens, Foresight Institute., February 9th, 2021.

Denvil Duncan, Justin Ross, and Coady Wing, Aliens and UFOs in the Social Sciences with Robin Hanson, Lunchtime Social Science, February 16th, 2021.

Corey Wolf, Dixon Hill, and Passer Langhaus, Nice Galactic Bulge, Sweetcheeks, Briar Patch Observatory, February 21st, 2021.

A Simple Model of Grabby Aliens, Future of Humanity Institute, March 5th, 2021.

Matt Carter, Joey Svendsen, God and Aliens with Robin Hanson, Bad Christian Podcast, March 24th, 2021.

3D Sim Movie of Model

Coverage and discussion

Shtetl-Optimized, January 30th, 2021.

Reddit, January 30th, 2021.

Hacker News, January 31st, 2021.

Marginal Revolution, February 2nd, 2021.

Mind The Post, February 7th, 2021., February 11th, 2021.

Metaculus, February 20th, 2021.

Less Wrong, March 4th, 2021.

Finding The Portal, 2021.

Predict, June 9th, 2021.

Bloomberg Opinion, June 11th, 2021.

Hanson's core blog posts on grabby aliens

How Far To Grabby Aliens? Part 1., December 21st, 2020.

How Far To Grabby Aliens? Part 2., December 21st, 2020.

Try-Menu-Combo Filter Steps, January 3rd, 2021.

Why We Can’t See Grabby Aliens, January 8th, 2021.

An Adventure, February 2nd, 2021.

Humans Are Early, February 3rd, 2021.

Hail S. Jay Olson, February 21st, 2021.

SETI Optimism is Human Future Pessimism, March 7th, 2021.

Power Laws Approximate Appearance, March 26th, 2021.

Are Expansive Aliens Obvious?, March 27th, 2021.

Given Our Date, Is Sun Birth Late?, March 27th, 2021.

Hanson's speculative blog posts on aliens

Try-Try or Try-Once Great Filter?, December 3rd, 2020.

Searching For Eden, December 5th, 2020.

If Aliens Are Near, December 8th, 2020.

The Long Term Future of History, December 23rd, 2020.

What Is At Stake?, February 19th, 2021.

Non-Grabby Legacies, March 8th, 2021.

Do Foo Fighters Show Our Snafu Fubar Future?, March 12th, 2021.

UFO Stylized Social Facts, March 28th, 2021.

Explaining Stylized UFO Facts, March 30th, 2021.

To Beat Aliens, We Must Become Aliens, April 5th, 2021.

UFOs Show Govt. Competence As Either Surprisingly High Or Low, May 5th, 2021.

When The Tabooed Taboo, May 15th, 2021.

Panspermia Siblings, May 20th, 2021.

UFOs and Status, June 4th, 2021.

On UFOs-As-Aliens Priors, June 8th, 2021.

website content by Daniel Martin and Robin Hanson

built by William Eliot